Over the years, Ethernet has become the most commonly used method for local area networking. The IEEE 802.3 group, the originator of the Ethernet standard, has developed an extension to the standard, known as IEEE 802.3af, that defines supplying power over Ethernet cabling. The IEEE 802.3af standard defines a PoE system that involves delivering power over unshielded twisted-pair wiring from a Power Sourcing Equipment (PSE) to a Powered Device (PD) located at opposite sides of a link. Traditionally, network devices such as IP phones, wireless LAN access points, personal computers and Web cameras have required two connections: one to a LAN and another to a power supply system. The PoE system eliminates the need for additional outlets and wiring to supply power to network devices. Instead, power is supplied over Ethernet cabling used for data transmission.
As defined in the IEEE 802.3af standard, PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device. A PD is a device that is either drawing power or requesting power. PDs may be associated with such devices as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers and HVAC thermostats.
The main functions of the PSE are to search the link for a PD requesting power, optionally classify the PD, supply power to the link if a PD is detected, monitor the power on the link, and disconnect power when it is no longer requested or required. A PD participates in the PD detection procedure by presenting a PoE detection signature defined by the IEEE 802.3af standard.
If the detection signature is valid, the PD has an option of presenting a classification signature to the PSE to indicate how much power it will draw when powered up. Based on the determined class of the PD, the PSE allocates the required power to the PD. However, certain amount of allocated power is not delivered to the PD due to a power loss over an Ethernet cable between the PSE and PD
The IEEE 802.3af standard describes power distribution over the Ethernet by using the common mode voltage between 2 sets of twisted pairs within the CAT-5 cable. Current flows from the PSE to the PD on one twisted pair and return to the PSE on the other twisted pair. FIG. 1 illustrates a PoE system 10 described in the 802.3af standard. This system includes a PSE 12 that provides power to a PD 14 over the Ethernet cable having four twisted pairs of conductors—data pairs 16 and 18 and spare pairs 20 and 22. The data pairs 16 and 18 are respectively provided between data transformers 24 and 26 on the PSE side and data transformers 28 and 30 on the PD side. These data transformers may be used for connecting physical layer (PHY) devices involved in transmission (Tx) and reception (Rx) of data over the Ethernet.
The 802.3af standard indicates that the PSE 12 may be placed in 2 locations with respect to the Ethernet link segment. In particular, a PSE defined as an endpoint PSE may be arranged within data terminal equipment (DTE) or a repeater having a media dependent interface (MDI) that supports data transmission. Another type of a PSE defined as a midpoint PSE may be located within the link segment that is distinctly separate from the MDI and is between the MDIs.
The 802.3af standard indicates that Alternative A or Alternative B may be used for transferring power over the Ethernet. Alternative A involves transferring power over the data pairs 16 and 18, and usually is used for supplying power from endpoint PSEs. Alternative B provides transferring power over the spare pairs 20 and 22, and usually is used for supplying power from midpoints PSEs.
Propagation of power over a long Ethernet cable may result in substantial power loss. Currently, a PSE must allocate to a PD additional power to compensate for the maximum possible power loss over the Ethernet cable. However, it would be desirable to enable the PSE to determine how much power is being actually lost in the cable and to allocate additional power to compensate for the actual power loss, instead of the maximum possible power loss. To determine actual power loss, a cable resistance measuring mechanism is needed to measure the actual resistance of twisted pairs used for transferring power from the PSE to the PD. The PoE system treats a pair of wires connected to the same transformer winding as a single conductor. Therefore, to access power loss over the cable, the round trip DC resistance of a twisted pair should be determined.
A conventional cable resistance measuring scheme in a PoE system requires resistance measuring capabilities on both PSE and PD sides. For example, to support cable resistance measurements, a PD may include circuitry that provides short circuiting of the wires during the measurements. However, PDs may be any power consuming devices such as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers, HVAC thermostats, factory automation equipment, ID scanners, security systems, credit card terminals, and keyless entry systems. These devices do not have built-in capabilities for measuring resistance of the Ethernet cable that delivers power from the respective PSE. Therefore, it would be desirable to make cable resistance measurements without requiring cable resistance measurement capabilities on the PD side.
In addition, power interface circuitry provided between the PD and the Ethernet cable includes some elements, such as diode circuits, that have a considerable impact on cable resistance measurement results. Therefore, it would be desirable to develop a cable resistance measurement scheme that eliminates measurement errors caused by the power interface circuitry.
Moreover, to allow the PSE to dynamically allocate additional power based on the actual power loss over the cable, the cable resistance needs to be monitored while power is being delivered from the PSE to the PD over the cable. Therefore, it would be desirable to develop a cable resistance measuring scheme that does not interfere with the power delivery.